"But Olenna said they weren't the best fighters, and earlier in the series she said the Tyrell house words were shitty compared to the hardcore Starks or Greyjoys" is an actual excuse I heard for that shit when it happened.

House Tarly was but just one vassal of house Tyrell, a strong vassal for sure, but one of tens of vassals so it doesnt make sense for him to have half the army, and why would Olenna (one of the smartest characters in the show) leave that much power in the hand of just one house and one commander, especially one who answered Cersei's call to go to the capital. Furthermore, Highgarden is a strong castle, they should have easily been able to hold a siege till they ask for reinforcements from Dany.

It was just a stupid plot device that only aimed to nerf Dany without much sense, same as Euron bullshit